* The first major impact event to be clearly recorded took place in 1908, when a object leveled an expanse of Siberian forest. Of course, it wasn't really the first such event, but it was the first to be properly seen for what it was. Since that time, other historical impact events have been recognized for what they are.
* The only major impact of a celestial object on the Earth recorded in modern history occurred about a quarter after seven on the morning of 30 June 1908, when witnesses observed a huge fireball almost as bright as the Sun plunging across the Siberian sky, terminating in a huge explosion that registered on seismic stations all across Eurasia.
Surprisingly, there was little scientific curiosity about the impact at the time, and due to the subsequent occurrence of war, revolution, and civil war in Russia, it wasn't until the 1920s that anyone performed a serious investigation of what had happened in Siberia in 1908. In 1921, the Russian mineralogist Leonid Kulik visited the Podkamennay Tunguska River basin as part of a survey for the Soviet Academy of Sciences. Locals told him of the great blast, of huge stretches of forest being flattened, of people being blown over by the shock.
The reports were basically consistent with each other, and Kulik was able to persuade the Soviet government to fund an expedition to the Tunguska region. His group reached the "ground zero" of the "event" in 1927. Much to their surprise, there was no crater -- just a great region of scorched trees about 50 kilometers across. The trees pointed away from the center of the event, with a few still bizarrely standing upright at ground zero, their branches and bark stripped off.
Over the next ten years, there were three more expeditions to the area, and none of them discovered anything much different from what Kulik and his people had found. Kulik found a little "pothole" bog that he thought might be the crater, but after a laborious exercise in draining the bog, he found there were old stumps on the bottom -- which would have been surprising if the bog had been ground zero of the impact. Other "craters" would later be found, but were never linked to the impact.
Kulik did manage to arrange an aerial photographic survey of the area in 1938, a few years before his death as a Red Army officer in the Soviet Union's "Great Patriotic War" against Hitler. The aerial survey revealed that the event had knocked over trees in a huge butterfly-shaped pattern that provided information on the direction of the object's motion. Tidy Soviet experiments performed in the mid-1960s with model forests and small explosive charges slid downward on wires that duplicated this pattern suggested that the 1908 object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north, and exploded in mid-air.
Expeditions sent to the area in the 1950s and 1960s did find microscopic glass spherules in siftings of the soil. Chemical analysis showed that the spherules contained high proportions of nickel and iridium, which are found in high concentrations in meteorites, indicating extraterrestrial origin. However, even this clue could not pin down the nature of the object precisely.
* Many ideas have been proposed for what happened at Tunguska, such as an impact of an antimatter meteor, the passage of a tiny black hole, or even the catastrophic destruction of a nuclear-powered alien spacecraft. There has never been much evidence for such exotic ideas, and simpler theories were available.
In 1930, the British astronomer Fred Whipple suggested that the Tunguska event was the impact of a small comet, which vaporized itself in the explosion and left no obvious trace. Comets have traditionally been seen as "dirty snowballs" of ices and dust; modern examination by space probes has shown them to be more dust than ice, suggesting that the name "snowy dirtball" would be more appropriate, and also shown that they are low-density objects, full of voids, with one astronomer calling them "cosmic dust bunnies". A comet would be quickly destroyed by an impact with the Earth's atmosphere. The idea of a comet impact was reinforced by the fact that there were "skyglows" in the evenings across Europe for several days after the impact, obviously caused by dust dispersed through the upper atmosphere.
The comet idea remained popular for over 50 years, with some astronomers speculating that it might have been a piece of the short-period comet Encke. Materials from Encke apparently make up the stream of sky junk that create the "Beta Perseid" meteor shower, and the Tunguska event coincided with a peak in that shower. However, in 1983 an astronomer named Zdenek Sekanina, of the US National Aeronautics & Space Administration's Jet Propulsion Laboratory (NASA JPL), published an article that undermined the comet theory. Sekanina pointed out that eyewitness accounts and other evidence point only to one explosion, and that the object passed through the atmosphere at a shallow angle, remaining intact to an altitude of 8.5 kilometers. A dirty snowball of ice and gases would have not got that far in one piece.
Sekanina proposed that the object was a stony "chondritic" asteroid that rammed through the atmosphere until pressures and temperatures reached a point that caused it to abruptly disintegrate in a huge explosion, something like what would happen on a much smaller scale to an aspirin pill smashed with a hammer. The destruction was so complete that no remnants of substantial size survived. The material scattered into the upper atmosphere from the event would have caused the skyglows.
Sekanina's theory was appealing, but it was based on very limited information. Said one critic: "You can't make a sophisticated model from poor data." Sekanina admitted there was "a lot of handwaving" in his ideas. The comet theory still has its partisans, who point out that chemical analyses of the area have showed it to be enriched in cometary material, and suggest that the comet might have been extinct and had formed a tough "mantle" that allowed it to penetrate the atmosphere. In the absence of conclusive evidence the debate seems likely to continue, but at least nobody seriously thinks it was a UFO.
* Although the Tunguska impact was both spectacular and unparalleled in any historical record, it no longer seems as unique and unusual as it once did. We now know that Earth impacts, fairly big ones, are happening all the time. The late Eugene Shoemaker of the US Geological Survey came up with an estimate of the rate of Earth impacts, and suggested that an event about the size of the nuclear weapon that destroyed Hiroshima occurs about once a year. Such events would seem to be spectacularly obvious -- but they generally go unnoticed, for a number of reasons:
Some have been observed, such as the Revelstoke fireball of 1965, which occurred over the snows of northern Canada. A particularly interesting fireball was observed moving north over the Rocky Mountains from the US Southwest to Canada on 10 August 1972, and was filmed by a tourist at the Gran Teton National Park in Wyoming with an 8-millimeter color movie camera. The object was in the range of size from a car to a house and should have ended its life in a Hiroshima-sized blast, but there was never any explosion, much less a crater. Analysis of the trajectory indicated that it never came much lower than 58 kilometers above the ground, and the conclusion was that it had grazed the Earth for about 100 seconds, then skipped back out of the atmosphere to return to its orbit around the Sun.
Another fireball blew up over the Australian town of Dubbo in April 1993, shaking things up a bit but causing no harm. On the dark morning hours of 18 January 2000, a fireball exploded over the town of White Horse in the Canadian Yukon at an altitude of about 26 kilometers, lighting up the night like day and bringing down a third of the Yukon's electrical power grid, due to the "electromagnetic pulse" created by the blast. The meteor that produced the fireball was estimated to be about 4.6 meters in diameter and with a weight of 180 tonnes.
At midmorning on 15 February 2013, a fireball streaked over the sky of the Chelyabinsk region in central Russia, finally breaking up in a stratospheric explosion. The fireball lit up the sky like a second Sun; many residents went to their windows to see what was going on, to be cut up when the shockwave arrived and shattered the glass into shards. At least one small crater was found, created by the fragments of the meteor's disintegration. Many video recordings were made of the event by dashboard cameras, common in Russia.
Analysis suggested the meteor was about 18 meters in diameter, with a mass of about 11,000 tonnes, and hit the Earth's atmosphere at a speed of about 68,000 KPH (42,000 MPH). It exploded at an altitude of about 24 kilometers, generating a blast equivalent to over 400 kilotons of TNT -- about 30 times more powerful than the nuclear weapon that destroyed Hiroshima. It was the largest known meteor strike since the Tunguska event. The Chelyabinsk meteor was clearly a stony object, not a rarer iron-nickel one, which would have penetrated the atmosphere more deeply and caused far more damage.
As mentioned, many impact events occur without being observed by anyone on the ground. Between 1975 and 1992, American missile launch early warning satellites picked up 136 major explosions in the upper atmosphere. The Tunguska event was about a thousand times more powerful than such events. Shoemaker estimated that one of such magnitude occurs about once every 300 years -- though a more recent study, published by MIT Lincoln Laboratory researcher J. Scott Stuart in 2003, suggested that Shoemaker's estimate was exaggerated by an order of magnitude, and that the interval is more like 3,000 years. That is still not that long an interval; it is a somewhat nerve-wracking question to consider when the next "Big One" will be, and more to the point, where.
BACK_TO_TOP* The Tunguska fireball was the only major impact event to be witnessed and recorded, but there is plenty of evidence for other major impact events.
Settlers in the American West had found a great crater in the barrens of the Colorado Plateau in northern Arizona state, about 55 kilometers east of Flagstaff, its rim making it visible above the flatlands around it. Some geologists believed it was a volcanic crater, but in 1905 an engineer and businessman named Daniel M. Barringer suggested it was the result of the impact of a large iron-metallic meteorite. Later research would prove him right, since the crater was lined with materials showing the effects of the enormous pressures and high temperatures associated with an impact event.
Impacts produce distinctive "shock-metamorphic" effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include:
Barringer had hoped to mine the site for iron, but the meteorite's pieces had been scattered far and wide, and his scheme came to nothing. However, in some compensation, the formation was named the "Barringer Meteor Crater" after him.
The impact event that formed the Barringer Crater took place about 50,000 years ago. At that time, the climate on the Colorado Plateau was cooler and damper. The area was a grassland dotted with woodlands, and inhabited by woolly mammoths, giant ground sloths, and camels.
One day an iron-metallic meteor about 50 meters across fell from the sky, burning much brighter than the Sun, at a speed faster than 40,000 kilometers per hour (KPH). The object slammed into the ground, producing a massive explosion that was about three times more powerful than the Tunguska event. The explosion dug out 175 million tonnes of rock, leaving a crater about 1,200 meters across and 170 meters deep. The energy of the impact propagated as a hemispherical shock wave that blasted the rock downward and outward from the point of impact, forming the crater.
At ground zero, the impact melted everything that it did not vaporize, and transformed carbon minerals into diamonds; 30-tonne blocks of limestone were tossed outside the crater's rim. The shock of impact sped through the ground, resulting in a magnitude 5.5 earthquake. Everything within a radius of three to four kilometers was killed immediately. The fireball that formed should have scorched everything within a radius of ten kilometers. A shock wave, moving out at 2,000 KPH, leveled everything from 14 to 22 kilometers, dissipating to hurricane-force winds that persisted to a radius of 40 kilometers. Despite this destruction, the Barringer impact did not throw up enough dust to affect the Earth's climate. The area was likely recolonized by the local flora and fauna within a century.
* Once the Barringer Crater was identified for what it was, other impact craters were gradually discovered, and dozens are now known. The older they are, the more likely they are to be buried, only being discovered by drill cores.
One of the oldest known impact craters is the Vredefort Crater in South Africa, which is about 300 kilometers across and two billion years old. It is only visible as a semicircle of low hills near Johannesburg. It is believed to have been created by an impactor about 20 kilometers in diameter.
One of the oldest craters that can be readily seen as such is the Manicouagan Crater in southern Quebec, visible from orbit as a relatively neat ring reservoir about 72 kilometers in diameter. It was created about 214 million years ago by the impact of an object about 5 kilometers in diameter.
It wasn't until the 1980s that Chesapeake Bay, which breaks up the US state of Maryland, was determined by drilling to be the remnant of an impact crater. About 35 million years ago, a giant meteor slammed into the Earth at what is now the mouth of the bay, blasting a crater almost 90 kilometers in diameter, incinerating much of what is now the US East Coast, and sending tsunamis across the Atlantic. The geology of the crater appears to be contributing to the unusually rapid rise of the sea on the shores of locales in the area, such as Norfolk, due to the continued settling of the material that eventually mostly filled up the crater -- and the subsidence of ground that was pushed up around its rim.
There is another, easily visible, impact crater in Quebec, inside Pingualuit National Park in the north of the province. The area is inaccessible and nobody noticed the crater lake until aerial photos were taken of it in the 1940s. A diamond prospector named Frederick W. Chubb teamed up with geologist V. Ben Meen to visit the site in 1950; Chubb thought the crater might be volcanic in origin, meaning there could be diamonds there, but Meen didn't think so. Meen returned to "Chubb Crater", as it was originally named, with a research team in 1951 and 1954, though it wasn't until the early 1990s that the Pingaluit Crater was officially recognized as the result of an impact event. The crater is 3.44 kilometers in diameter -- much bigger than the Barringer Crater -- and is estimated to be about 1.4 million years old.
Another recent crater was discovered in 2015, when Kurt Kjaer -- a geologist at the Center for GeoGenetics at the Natural History Museum of Denmark -- and a colleague were inspecting a NASA map of Greenland, when they noticed an enormous circular depression on the Hiawatha Glacier at Greenland's northwest tip. The two looked it over and said: "What is THAT?" They quickly realized that it could be a meteor crater.
Kjaer's research team got in touch with NASA scientists, who provided them with radar scans of parts of the area in question, obtained by aircraft. The radar could penetrate the ice, and the images revealed the rim of a crater. It wasn't clear, however, if it was an impact crater or a volcanic crater. The distinguishing feature of an impact crater is a central peak -- more on this later -- which shouldn't be found in a volcanic caldera. Kjaer's team scanned what they believed should be the center of the crater with penetrating radar carried on an aircraft, and found the central peak.
The team's analysis showed that the Hiawatha crater is nearly 300 meters deep and 32 kilometers in diameter. The bowl of the crater presses right against the edge of the Hiawatha glacier, its semi-circular appearance hinting at the crater. Emerging from the semicircle is a white tongue of ice, a large river containing sediments from the bottom of the ice sheet. The team flew in by helicopter and collected samples, the samples including pieces of highly shocked quartz, a clear marker of an impact. The sediments also had high concentrations of nickel, cobalt, chromium, gold, and platinum, an indicator that the meteorite was made of iron.
The fact that the crater is buried under ice makes it hard to examine. They believe the meteor that formed the crater was about a kilometer in diameter. Rock debris could have reached as far away as Europe and America, with the released steam, a greenhouse gas, locally warming Greenland, and melting more ice.
The crater is no older than a few million years, possibly even less than a million years old. It is tempting to think the impact occurred at the start of the "Younger Dryas", 12,800 years ago, when temperatures dropped by 8 degrees Celsius (14 degrees Fahrenheit) in parts of the Northern Hemisphere, and stayed that way for a millennium. However, available Greenland ice core samples, which should have a layer of debris, don't back up that idea. The researchers intend to obtain more samples.
* In hindsight, the Barringer Crater looks so much like the result of an impact that it is hard to imagine anyone could think it was anything else, but not all remnants of impact events are so obvious.
In 1990, Captain Ruben Lianza of the Argentine Air Force, an amateur astronomer, provided a report to an astronomy publication that included aerial pictures of a set of odd teardrop-shaped depressions near the city of Rio Cuarto in north-central Argentina. The depressions seemed very similar to the sets of craters produced in laboratory simulations of impacts taking place at low angles. Such features exist on the Moon, Mars, and Venus, but had not been seen on Earth up to that time.
The depressions had been long known to Argentine geologists, but until Lianza came along, nobody had seriously investigated them. Samples of materials obtained from the depressions indicated the presence of shocked materials, as well as pebbles that were clearly of meteoritic origin. A team of American researchers went to Argentina to investigate, collaborating with Lianza and Argentine academics to study the strange depressions.
There were ten depressions, four of them of substantial size. One crater, named the "Drop", was about 200 meters wide and 600 meters long. Two more large craters, the "Eastern Twin" and "Western Twin", both about 700 meters wide and 3.5 kilometers long, were located 5 kilometers to the northeast. Another major crater, the "Northern Basin", about half as big as one of the Twins, was sited 11 kilometers further to the northeast. The long axes of the craters all pointed to the northeast.
The craters were apparently due to a grazing impact of a set of objects at a very low angle, which calculations show to be a rare occurrence. Most impacts will strike at an angle no more than 45 degrees from the vertical, and the impact craters will always be close to circular, since the shock wave that results from the impact propagates symmetrically. A grazing impact, however, will form an elliptical crater, with sprays of debris that look like butterfly wings. This has been confirmed by high-velocity guns used for impact experiments, and more recently by computer simulations. On impact, the object may shed chunks of itself that fly further downrange to perform secondary impacts.
Models of the Rio Cuarto event suggest that the object struck at an angle of no more than 15 degrees from the horizontal, with the impact itself having 10 times more explosive energy than the Barringer event and 30 times more than the Tunguska event. Although the age of the craters has not yet been determined precisely, it is believed they are about 10,000 years old.
The object came in from the northeast, bright as a second Sun. The object hit ground at the Northern Basin, creating a mountain of fire about 10 kilometers wide and 50 kilometers long, and scattered off pieces that went downrange to form the Twins and the Drop. The fireball incinerated all life downrange in a firestorm with a parabolic-shaped footprint that created hurricane-force winds, erasing the butterfly-shaped pattern of debris characteristic of such low-angle strikes.
The object was clearly a "carbonaceous chondritic" asteroid, largely made up of simple carbon compounds and resembling something like a big lump of soot. The impact probably released huge clouds of toxic carbon monoxide that killed off wildlife in the area, assisted by heavy concentrations of toxic nitrous oxides created through ionization by the object's fiery passage through the atmosphere. It is likely the impact resulted in serious atmospheric effects and may have had a short-term effect on global climate.
More recent research has disputed that the Rio Cuarto craters are actually due to an impact event, instead being artifacts of wind erosion. The presence of shocked quartz grains and such does suggest an impact, but it is possible the original craters were distorted by the wind.
In the mid-1980s, satellite photography revealed a neat circular structure in the Bolivian lowlands about 8 kilometers across that seemed to be an impact crater. The region was somewhat inaccessible, but expeditions finally confirmed that the structure was in fact an impact crater, produced by the impact of a body about 30,000 years ago with the yield of about a gigaton of TNT.
BACK_TO_TOP* The first known impact to have taken place during human history obliterated the ancient town of Tall el-Hammam, in what is now Jordan, which came to an abrupt and cataclysmic end some 3,600 years ago. Excavations at Tall el-Hammam began in 2005, with archaeologists quickly realizing that something disastrous had happened to the town, with buildings smashed, the inhabitants blasted apart, and intense heat scorching the area.
Closer examination was that the damage indicated intense heat, with melted rocks and metal artifacts -- which wouldn't have happened if the town had simply burned down. Could it have been a meteor strike? In 2014, the excavators got in contact with the Comet Research Group (CRG), an international collaboration that investigates "space strikes".
Over the next seven years, a CRG team under Malcolm LeCompte -- of Elizabeth City State University in North Carolina -- visited the site three times to collect samples. In a layer of sediment dating back to the time of the strike, they found shocked quartz grains. The heat and pressure had even converted wood and plants into tiny diamonds. In addition, the researchers found spherules, made up of vaporized iron and sand, with tell-tale traces of platinum, iridium, and osmium.
The pattern of damage suggested the blast was to the southeast of the town, with LeCompte estimating that it was in the range of 5 to 30 megatons, far more powerful than the blast that destroyed Hiroshima. Sediments from the time of the blast have a high level of salt, possibly due to depositions from the nearby Dead Sea caused by the blast, which made growing crops in the area impractical for centuries afterward. LeCompte has casually suggested the disaster was the basis for the Sodom & Gomorrah story.
In 2004, German researchers announced that studies had shown a large body hit the Earth's atmosphere over southern Bavaria in 200 BC. The size of the body was estimated at about 1.1 kilometers and the impact produced an enormous explosion that dwarfed the Tunguska blast. However, the damage was modest relative to the size of the object, since the object broke up at high altitude, leading the researchers to believe it was a comet. Large fragments did fall to Earth over an elliptical area, leaving a pattern of craters -- the largest now being Lake Tuettensee, which is about 370 meters across. The original actual crater appears to be about twice that big. Roman authors of the time do make references to tales of fires in the sky and stones falling from the sky.
University of Cincinnati (UC) researchers similarly found traces of a comet impact over North America 1,500 years ago, found at 11 sites related to the Hopewell tribal culture of the era, which was found over a large area of what is now the US Northeast. The 11 sites were in three US states along the Ohio River Valley. UC archaeologists found an unusually high concentration and diversity of meteorites at Hopewell sites compared to other time periods. The meteorite fragments were identified from the telltale concentrations of iridium and platinum they contained. The researchers also found a charcoal layer that suggests the area was exposed to fire and extreme heat. UC anthropology professor Kenneth Tankersley said:
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These micrometeorites have a chemical fingerprint. Cosmic events like asteroids and comet airbursts leave behind high quantities of a rare element known as platinum. The problem is platinum also occurs in volcanic eruptions. So we also look for another rare element found in nonterrestrial events such as meteorite impact craters -- iridium. And we found a spike in both, iridium and platinum.
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The surviving Hopewells collected the meteorites and used the metal in jewelry and other artifacts. The airburst affected an area bigger than New Jersey, setting fires across 23,850 square kilometers sometime between the years 252 and 383 CE. This coincides with a period when 69 near-Earth comets were observed and documented by Chinese astronomers, and witnessed by Native Americans as told through their oral histories. According to Hopewell, who has tribal roots, Algonquin and Iroquoian tribes, descendants of the Hopewell, spoke of a calamity that befell the Earth. Similar stories are found among the Miami, Shawnee, Ottawa, and Wyandot. The impact was disastrous to the Hopewell, with its longer-term ecological effects being ruinous as well.
Another major impact apparently occurred much more recently. The vast desert wasteland of southern Saudi Arabia known as the "Empty Quarter", or "Rub' al Khali" in Arabic, is one of the most desolate places on Earth. In 1932, a British explorer, Harry Saint John "Abdullah" Philby, father of Red spy Kim Philby, was hunting for a city named "Ubar", that the Koran claimed had been destroyed by God for defying the Prophet.
Philby mistranslated the name of the city as "Wabar" -- which in a way was fortunate, because he found something else that deserved a different name. After a month's journey through the wastes that was so harsh that even some of the camels died, Philby found a patch of ground about a half a square kilometer in size, littered with chunks of white sandstone, black glass, and chunks of iron. There were two large circular depressions partially filled with sand. He came back with one of the chunks of iron; analysis showed it to be about 90% iron and 5% nickel, with the rest consisting of various elements, including an unusually high concentration of iridium. This implied that the "Wabar" site was a meteorite impact area. Later research located the town of Ubar elsewhere, but Philby's Wabar site remained intriguing.
In 1994, the Zahid Tractor Corporation, a Saudi dealer of the "Hummer" off-road vehicle, decided to stage a publicity stunt of the vehicle by driving several of them across the Empty Quarter in the dead of summer -- few ever went deep into the Empty Quarter in the summer and came back alive. A US Geological Survey scientist, Jeffrey C. Wynn, was invited to come along. Zahid sponsored a total of three trips into the Empty Quarter in 1994 and 1995, and Wynn went on all of them. Even with modern technology, the trip was a difficult one. Not only were conditions harsh, but the Wabar site was tricky to find, since it sits in the middle of an enormous dune field that has few fixed landmarks.
The Wabar site covers about 500 by 1,000 meters and features three prominent, roughly circular craters. Two were reported by Philby, and measure 116 and 64 meters wide. The third was discovered on the Zahid expeditions and is 11 meters wide. They are all nearly full of sand. The surface of the area partly consisted of "impactite" rock, a whitened coarse sandstone, and was littered with black glass slag and pellets. The impactite had a laminated appearance and featured a form of shocked quartz known as "coesite", and seemed clearly the product of an impact event. The impact did not penetrate to bedrock.
The presence of iron fragments at the site also pointed to a meteorite impact, since there are no iron deposits in the region. The iron was in the form of buried fist-sized cracked balls and smooth fragments found on the surface. The largest fragment was recovered in a 1965 visit to Wabar and weighs 2.2 tonnes. It is known as the "Camel's Hump" and is on display at the King Saud University in Riyadh. The sand was turned into black glass near the craters, and pellets of the glass are scattered all over the area. The glass is about 90% local sand and 10% meteoritic iron and nickel.
The layout of the impact area suggests that the body fell at a shallow angle, and was moving at typical meteorite entry speeds of 40,000 to 60,000 KPH. Its total mass was more than 3,500 tonnes. The shallow angle presented the body with more air resistance than it would have encountered at a steeper angle, and it broke up in the air into at least four pieces. The biggest piece struck with an explosion about as powerful as that produced by the bomb that leveled Hiroshima.
One analysis of glass fragments suggested the Wabar impact took place thousands of years ago, but the fact that the craters have filled up considerably since Philby visited them suggests their origin is much more recent, and other chemical analyses suggest the impact site is no more than a few centuries old. Arab reports of a fireball passing over Riyadh, variously reported as occurring in 1863 or 1891, indicate the impact may have occurred very recently. Fragments scattered from the path of this fireball match samples found at the Wabar site.
BACK_TO_TOP* Centuries ago, the Western vision of the past saw an Earth that had been created a few thousand years ago, and had been shaped since that time by a number of global cataclysms. This view gradually gave way to the consensus that the Earth was several billion years old, and that its features reflected the slow processes of gradual change. Since 1970, this view has gradually expanded to accommodate the evidence that the Earth has in fact gone through periods of abrupt and catastrophic change, in some cases due to the impact of large asteroids and comets on the planet. A few of these impacts may have caused massive climate change and the extermination of large numbers of plants and animals.
The fact that this modified view of the Earth's history did not emerge until recently seems surprising. Although it might be assumed that a major impact on the Earth would leave behind absolutely unmistakeable evidence, in fact the gradual processes that change the surface of the Earth tend to cover the effects of impacts. Erosion by wind and water, deposits of wind-blown sand and water-carried sediment, and lava flows in due time tend to obscure or bury the craters left by impacts. However, some evidence remains, and over 150 major craters have been identified on the Earth. Studies of these craters have allowed geologists to find the remaining traces of other craters that have mostly been obliterated.
* Daniel Barringer was one of the first to identify a geological structure as an impact crater, but at the time his ideas were not widely accepted, and when they were, there was no recognition of the fact that Earth impacts are common in geological terms.
In the 1920s, the American geologist Walter H. Bucher studied a number of craters in the US. He concluded they had been created by some great explosive events, but believed they were the result of massive volcanic eruptions. However, in 1936, the geologists John D. Boon and Claude C. Albritton JR revisited Bucher's studies and concluded the craters he studied were probably formed by impacts.
The issue remained more or less speculative until the 1960s. A number of researchers, most notably Gene Shoemaker, conducted detailed studies of the craters that provided clear evidence that they had been created by impacts, identifying the shock-metamorphic effects uniquely associated with impacts. Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Observatory in Canada and Wolf von Engelhardt of the University of Tuebingen in West Germany began a methodical search for "impact structures". By 1970, they had tentatively identified more than 50.
Their work remained controversial, but the American Apollo Moon landings, which were in progress at the time, provided evidence of the rate of impact cratering on the Moon. Processes of erosion on the Moon are minimal and so craters persist almost indefinitely. Since the Earth could be expected to have roughly the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters.
* The age of known impact craters on the Earth ranges from a few thousand to almost two billion years -- the Vredefort Crater is possibly the oldest -- though few older than 200 million years have been discovered since geological processes tend to obliterate older ones. They are also selectively found in the stable interior regions of continents. Few underwater craters have been discovered because of the difficulty of surveying the sea floor; the rapid rate of change of the ocean bottom; and the "subduction" of the ocean floor into the Earth's interior by processes of continental drift. Current estimates of the rate of cratering on the Earth suggest that from one to three craters with a width greater than 20 kilometers are created every million years. This indicates there are far more relatively young craters on the planet than have been discovered to date.
An asteroid falls onto the Earth at a speed of about 40,000 to 60,000 KPH. If the object weighs more than 1,000 tonnes, the atmosphere does not do much to slow it down -- though smaller bodies can be substantially slowed by atmospheric drag, since they have a higher ratio of surface area to mass. In any case, the temperatures and pressures on the object are extremely high. They can destroy chondritic or carbonaceous chondritic bodies before they ever reach ground, but iron-metallic asteroids have more structural integrity and can strike the surface of the Earth in a violent explosion.
The result is a crater. There are two forms, "simple" and "complex". The Barringer Crater in Arizona is a perfect example of a simple crater, a straightforward bowl in the ground. Simple craters are generally less than four kilometers across. Complex craters are larger, and have uplifted centers that are surrounded by a trough, plus broken rims. The uplifted center is due to the "rebound" of the earth after the impact. It is something like the ripple pattern created by a drop of water into a pool, frozen into the Earth when the melted rock cooled and solidified.
In either case, the size of the crater depends on the material in the impact regions. Relatively soft materials yield smaller craters than brittle materials. Erosion and other geological activities quickly hide impact craters on the Earth. The Barringer Crater is in superlative shape, but it is only about 50,000 years old.
Some volcanic features can resemble impact craters, and brecciated rocks are associated with other geological formations besides impact craters. The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in the uplifted center of a complex crater, however.
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